The present invention relates to an amplifier circuit and, particularly, to a preamplifier circuit for use in a magnetic recording and reproducing apparatus such as video tape recorder.
In the magnetic recording and reproducing apparatus, the frequency characteristics of a reproduced video signal are generally determined by resonance characteristics defined by an inductance. As such, capacitance of a video head circuit and it is very difficult to obtain desired frequency characteristics therefor. In order to try to overcome this problem, it has been usual to improve the resonance characteristics of the video head circuit by using a damping resistor. FIGS. 1 and 2 show examples of conventional circuits for this purpose, noting that FIG. 1 is disclosed in U.S. Pat. No. 3,513,267. In these figures, a letter L depicts an equivalent inductance of a video head when viewed from an input side of a preamplifier whose gain is A, and C is a parallel capacitance including a stray capacitance of the video head. The high cutoff frequency of a reproduced video signal is substantially determined by a resonance frequency defined by L and C according to the following equation EQU fo =1/2.pi..sqroot.LC
In order to restrict such frequency characteristics due to resonance and to make a frequency characteristics flat within the video signal frequency band, damping resistors R2 and R1 are employed in the circuits shown in FIGS. 1 and 2, respectively. Among them, since the circuit in FIG. 1 includes the damping resistor R2 inserted into the circuit as a negative feedback resistor, it is possible to reduce the amount of thermal noise which is produced by the resistor. For this reason, the circuit shown in FIG. 1 has been frequently used in current video tape recorders.
In these prior art arrangement, when it is desired to make the frequency band of the reproduced video signal wider to obtain a higher video signal quality, it is necessary to increase the resonance frequency by reducing the inductance L or capacitance C of the video head. In order to reduce the inductance, it is necessary to reduce the number of turns of the head coil, resulting in a reduced induction voltage and a degraded S/N ratio. Alternatively, a reduction of the parallel capacitance is practically impossible since a large part of this is the stray capacitance of the head coil. Therefore, at present, it is almost impossible to obtain flat frequency characteristics for a video signal over a substantially wider frequency range.